Process For Producing An Absorbent Polymer By Means Of Spread-Drying

ABSTRACT

The process according to the invention for production of an absorbent polymer comprising the following process steps: i) polymerization of an aqueous monomer solution to obtain a polymer gel; ii) comminution of the polymer gel to obtain a gel granulate; and iii) drying of the gel granulate, and is characterized in that the gel granulate after step ii) has a spread behavior. The production process of an absorbent polymer distinguishes itself by a particularly efficient drying step iii), which enables a particularly gentle and uniform drying of the gel granulate. The obtained absorbent polymers and products and items produced therefrom have a particularly uniform product quality and constant physical-chemical properties.

This application is a national stage application under 35 U.S.C. 371 ofinternational application No. PCT/EP2005/004228 filed Apr. 20, 2005, andclaiming priority to German Application No. DE 10 2004 019 264.2 filedApr. 21, 2004, the disclosures of which are expressly incorporatedherein by reference.

The invention relates to a process for the production of an absorbentpolymer, an absorbent polymer, a composite, a process for the productionof a composite, chemical products comprising an absorbent polymer, a useof an absorbent polymer and of a composite, and a device for comminutingand drying of a polymer as well as a polymerization device.

For a formation of so-called “absorbent” polymers, a polymerization ofvarious types of normally water-soluble monomers, often also togetherwith water-insoluble co-monomers in the presence of crosslinkers isnecessary. The addition of the crosslinkers occurs during or after thepolymerization. Absorbent polymers of this type are lightly crosslinked,water-insoluble hydrogel polymers, which have a high capacity for waterabsorption in the dry and in the substantially water-free state. Thiscan comprise a multiple of their own weight. By reason of the highabsorption capacity, absorbent polymers are suitable for incorporationinto water-absorbing structures and objects, such as, e.g. baby diapers,incontinence products or sanitary napkins. These absorbent polymers arealso described as “superabsorbers” in the literature. In this context,reference is made to Modern Superabsorbent Polymer Technology; F. L.Buchholz, A. T. Graham, Wiley-VCH, 1998.

The production of polymers of this type occurs in the presence ofsolvents, such as water, so that a drying step for drying of theobtained polymer gel is necessary. The type and method of drying has asignificant influence on the microscopic structure and the physical andchemical properties of the polymer produced, so that particular care inthe drying of the polymer is required.

The drying step represents a cost-intensive process step, because thewater must be removed from the polymer against osmotic capillary forcesand forces caused by dipole-dipole interactions and hydrogen bridgingbonds as well as adhesion forces. Commonly, a polymer to be dried willbe comminuted into a gel granulate, transferred as a layer onto aperforated belt and dried by means of a stream of air.

An increase in the efficiency of the drying effects a significant costsaving. On the other hand, the efficiency in the drying cannot occur atthe expense of a gentle and uniform drying. Previously known dryingprocesses often had the disadvantage that they led to an inhomogeneousdrying because of the variations in the thickness of the layer of gelgranulate to be dried and/or because of variations on the density of thegel granulate within the layer. An inhomogeneous drying has adisadvantageous effect on the product quality and the absorptionproperties of the absorbent polymer and of the further processingproducts containing this polymer, e.g. hygiene articles of all types.Furthermore, an inhomogeneous drying of the gel granulate leads tounsatisfactorily dried and therefore viscous gel granulate remainingadhered to the surfaces of the pulverization device connected followingthe drying device, which frequently leads to downtime of thepulverization device.

Furthermore, an inappropriate drying leads to caking of the particles ofthe polymer to be dried accompanied by formation of a cake.Inhomogeneities of this cake result in an uneven drying and theformation of very solid areas in this cake, through which air can passonly with difficulty. These areas must be further broken up, withsignificant mechanical stress on the polymer. A significant andundesirable formation of dust is associated herewith.

To solve these problems, JP 08/73518 proposes the continuousdetermination of the thickness of the gel layer on a belt dryer and theadaptation of the drying conditions to the thickness of the gel layer.U.S. Pat. No. 6,291,636 B1 proposes the separation of insufficientlydried granulate in a separation step occurring before the pulverizationstep.

The separation of insufficiently dried granulate proposed in U.S. Pat.No. 6,291,636 B1 is avoided according to the teaching of WO 03/051939A1, by adding a minimum amount of furfural to the acrylic acid used inthe polymerization. This process is, however, disadvantageous, sincefurfural does not only work as a polymerization inhibitor, but is alsotoxic, which is particularly questionable for the use of the polymers inhygiene articles.

In general, the present invention overcomes the disadvantages arisingfrom the state of the art.

In one aspect of the present invention, a process is provided accordingto which absorbent polymers can be produced, whereby a particularlyefficient and uniform drying of the gel material obtained asintermediate product is effected. This process should lead in as fewprocess steps as possible to superabsorbent polymers with uniformquality and with an amount as small as possible of toxicologicalimpurities.

In another aspect of the present invention, a process is providedaccording to which absorbent polymers or composites comprising polymersof this type can be produced, whereby the absorbent polymers have aparticularly uniform quality and physical-chemical propertiescorresponding to their use, in particular absorbent properties.

In an additional aspect of the present invention, an absorbent polymer,composites which comprise absorbent polymers of this type and chemicalproducts which comprise absorbent polymers of this type, which haveparticularly uniform physical-chemical properties and minimumtoxicological impurities, are provided.

In yet another aspect of the present invention, a device is providedwith which absorbent polymers with particularly uniform physicalproperties can be prepared cost-effectively.

In a further aspect of the present invention, the mechanical stress ofthe further processing devices connected following the polymerizationreactor is reduced, in order to reduce the probability of theirbreakdown and to prolong their length of operation.

Additionally, one aspect of the present invention lies in the provisionof a superabsorbent polymer which can be incorporated particularly wellinto hygiene articles, which influences advantageously the absorbentproperties of the hygiene articles and which is characterized by a smallloading of toxicological impurities.

These aspects of the present invention are provided by the process forproducing an absorbent polymer, by the absorbent polymer, by thecomposite comprising the absorbent polymer, by the process for producinga composite, by the chemical products comprising the absorbent polymeror the composite, by the use of the absorbent polymer, by the device forcomminuting and drying a polymer and by the polymerization device, asgiven in the respective independent claims. Advantageous embodiments andfurther developments, which can appear individually or in combination,are the object of the respective dependent claims.

The process according to the invention for production of an absorbentpolymer comprises the process step

i) polymerization of an aqueous monomer solution to obtain a polymergel,

ii) comminution of the polymer gel to obtain a gel granulate,

iii) drying of the gel granulate,

and is characterized in that the gel granulate

after step ii), for example, after step ii) and before the start of stepiii) or after step ii) and during step iii) or after step ii) and beforeand during step iii),

such as at a time, at which the gel granulate still has a water contentbetween about 20 and about 90 wt. %, such as between about 40 and about70 wt. % and such as between about 45 and about 65% and such as betweenabout 50 and about 60%, respectively based on the total weight of thegel granulate, and a temperature T_(GP) (GP=gel particle) within a rangeof about 20° C. to about 150° C., such as within a range from about 30°C. to about 100° C., such as within a range from about 40° C. to about80° C. and most such as within a range from about 50° C. to about 70°C.,

has a spread behavior according to the herein described test methods.

The start of step iii) is any point in time during the course of theproduction of the absorbent polymers at which the polymer gel obtainedafter the polymerization, after the comminution is heated, for example,by conduction of energy in the form of heat or in the form of radiation,such as IR-radiation, to a temperature of about 60° C., at least about100° C. in another aspect and at least about 150° C. in an additionalaspect, for example, heated so that a continuous reduction of the watercontent of the polymer and thereby a drying of the same is enabled.

The spread behavior is the attempt of the gel granulates to equilibratelayer thickness variations. In this respect, not only gravitationalforces but also elastic forces have an effect. For example, if the gelgranulate is carried on a horizontal surface, the gel granulatedistributes itself on this surface, whereby it displaces the tendency toequilibrate troughs and peaks and to effect a leveling of the gelgranulate. The elastic forces drive the gel granulate apart and theindividual gel granulate particles attempt mechanically to relax. If gelgranulate particles have mechanically relaxed, they behave like knowngel granulates. The elastic forces of the gel granulate particles lendthe gel granulate a living character.

The drying is simplified by the spread behaviour of the gel granulate,since it leads to a more even layer thickness. If, for example, a gelgranulate of this type is shaken for drying from a fall height betweenabout 20cm and about 80cm, such as with a fall height between about 30cm and about 50 cm, onto a drying belt (belt dryer), the spreadbehaviour of the gel granulate advantageously leads to the variations ofthe layer thickness of the gel granulate on the drying belt (averagelayer thickness on the drying belt lies between about 5 cm and about 20cm, in particular between about 10 cm and about 15 cm) amounting to lessthan about 30%, such as less than 20%, and such as less than 10%. Avariation of, for example, less than about 10% means that the height ofthe gel at the position with the smallest thickness of the gel deviatesby less than about 10% from the height of the gel at the position withthe largest thickness of the gel. By means of this equilibration of thelayer thickness, the gel granulate can be dried particularly uniformly.

Gel granulates of an absorbent polymer which commonly have a watercontent between about 20wt % and about 90wt %, such as between about 40and about 70 wt. %, such as between about 45 and about 65 wt. % and suchas between about 50wt % and about 60 wt % after the polymerization, canbe dried particularly efficiently because of the levelling effect of thespreading. The thus dried absorbent polymer has particularly evenphysical-chemical properties.

In one embodiment of the process according to the invention, at leastone, and in one aspect, all of the following characteristics arepresent:

-   -   a. the polymer gel is mechanically loosened after the        comminution in step ii) in the form of the gel granulate        obtained after the comminution and before the previously        described start of the drying step iii) and a loosened gel        granulate is obtained, whereby the loosened gel granulate has a        spread behaviour determined according to the herein described        test methods;    -   b. the polymer gel is first mechanically loosened during the        drying step iii), for example after at least about 5 wt %, such        as at least about 10 wt %, such as at least about 20 wt % and        such as at least about 30 wt % of the water contained in the        polymer gel before the start of the drying step iii) is        evaporated, and a loosened gel granulate is obtained, whereby        the loosened gel granulate has a spread behaviour according to        the herein described test method.

The above described loosening of the gel granulate in the embodiments a.and b. occurs, for example, at a time at which the gel particles stillhave the first cited water quantity and the first cited temperature.

The loosening of the gel granulate occurs thus either before or duringthe drying step iii). It is also possible to loosen the gel granulatebefore and during the drying process. The spread behaviour of the gelgranulate is here caused by the mechanical loosening. The looseningleads to a reduced bulk density of the undried gel granulate determinedaccording to ERT 460.1-99, of less than about 800 g/l in one aspect,less than about 750 g/l in another aspect, less than about 700 g/l in afurther aspect, less than about 650 g/l in an additional aspect, andless than about 600 g/l in yet a further aspect. In one embodiment, thebulk density of the undried granulate decreases in the cause of theloosening by at least about 1%, such as at least about 5%, such as atleast about 10%, such as by at least about 20% and such as at leastabout 30%. In another embodiment of the process according to theinvention the bulk density of the undried gel granulate decreases bymeans of the loosening by about 10% to about 50%, for example, by about20% to about 30%. In this context, a reduction of the bulk density of,for example, about 5%, means that the bulk density of the undriedgranulate after the loosening is smaller by at least about 5% than thebulk density before the loosening. In one particular embodiment of theprocess according to the invention, a reduction of the bulk density lieswithin a range from about 10% to about 50%, such as within a range fromabout 20% to about 40%.

In a further embodiment of the drying process according to theinvention, the spread behaviour of the gel granulate or of the loosenedgel granulate, after step ii), such as after step ii) and before thestart of step iii) or after step ii) and during step iii), or after stepii) and before and during step iii), such as at a time at which the gelgranulate still has the first mentioned water quantity and temperature,is characterized by at least one, such as each of the followingcharacteristics (δ1) to (δ4):

-   -   (δ1) A first compressibility index κ₁ of the gel granulate lies        between about 10% and about 40%, such as between about 15% and        about 30%, such as between about 18% and about 25%;        (δ2) a second compressibility index κ₂ of the gel granulate lies        between about 3×10⁻⁵ Pa⁻¹ and about 6×10⁻⁵ Pa⁻¹, such as between        about 3.5×10⁻⁵ Pa⁻¹ and about 5×10⁻⁵ Pa⁻¹, such as between about        3.9×10⁻⁵ Pa⁻¹ and about 4.3×10⁻⁵ Pa⁻¹;    -   (δ3) a first decompressibility index κ₁′ of the gel granulate        lies between about 3% and about 15%, such as between about 4 and        about 10%, such as between about 5% and about 8%;    -   (δ4) a second decompressibility index κ₂′ of the gel granulate        lies between about 3×10⁻⁵ Pa⁻¹ and about 7×10⁻⁵ Pa⁻¹, such as        between about 4×10⁻⁵ Pa⁻¹ and about 8×10⁻⁵ Pa⁻¹, such as between        about 5×10⁻⁵ Pa⁻¹ and about 6.5×10⁻⁵ Pa⁻¹.

Each of the above features represents further embodiments according tothe invention. Some exemplary combinations of figures include: δ16263,δ16264, δ16364, δ162, δ163, δ164, δ283, δ264, δ364, δ1, δ2, δ3, δ4,δ1δ2δ3δ4, particularly, δ1δ2δ3δ4.

The spread behavior which is advantageous for the drying of the gelgranulate is described by such compressibility and decompressibilityindices.

In a further embodiment of the process according to the invention, thegel granulate, after step ii), such as after step ii) and before thestart of step iii) or after step ii) and during step iii) or after stepii) and before and during step iii), such as at a point at which the gelgranulate still has the first mentioned water quantity and temperature,has at least one of the following properties:

(ε1) a cross section spread index Q determined according to the hereindescribed test method under a load with a total mass of about 1185 g ofat least about 3, such as at least about 5, such as at least about 7,such as at least about 7.5 and such as at least about 7.82, whereby thecross section spread index Q may be smaller than about 20;

(ε2) a cross section spread index Q determined according to the hereindescribed test methods under a load with a total mass of about 2175 g ofat least about 3, such as at least about 5, such as at least about 7,such as at least about 7.5 and such as at least about 7.56, whereby thecross section spread index Q may be smaller than about 20;

(ε3) a cross section spread index Q determined according to the hereindescribed test method under a load with a total mass of about 3185 g ofat least about 3, such as at least about 5, such as at least about 6,such as at least about 7 and yet such as at least about 7.23, wherebythe cross section spread index Q is may be smaller than about 20;

(ε4) a cross section spread index Q determined according to the hereindescribed test methods under a load with a total mass of about 6185 g ofat least about 3, such as at least about 4, such as at least about 5,such as at least about 6 and such as at least about 6.57, whereby thecross section spread index Q is may be smaller than about 20.

Each of the above features further represents an embodiment according tothe invention. The following are exemplary combinations of figures:ε1ε2ε3, ε1ε2ε4, ε1ε2, ε1ε3, ε1ε4, ε2ε3, ε2ε4, ε1, ε2, ε3, ε4, ε1ε2ε3ε4,particularly, ε1ε2ε3ε4.

In one embodiment of the process according to the invention, the gelgranulate has, after step ii), such as after step ii) and before thestart of step iii) or after step ii) and during step iii) or after stepii) and before and during step iii), such as at a point at which the gelgranulate still has the first mentioned water quantity and temperature,a spread time constant τ, determined according to the herein describedtest method under a load with a total mass of about 6185 g , of at leastabout 2 s, such as at least about 4 s, such as at least about 6 s andsuch as at least about 10s. In one aspect, the spread time constant τdoes not exceed a value of about 60 s.

The first κ₁ and the second κ₂ compressibility index respectively of thegel granulate, as well as the first κ₁′ and the second κ₂′decompressibility index respectively of the gel granulate are defined bycompression and decompression experiments described under test methods.

The determination of the cross section spread index Q and of the spreadtime constant T is also given in the test methods.

Advantageously, the gel is subjected in process step ii) to an at leastthree-step comminution

-   -   with a cutting unit, such as a knife, for cutting the gel into        flat gel strips, such as with a length within the range of about        5 mm to about 50 mm, such as from about 8 mm to about 40 mm and        such as from about 10 mm to about 30 mm, a height within the        range from about 1 mm to about 30 mm, such as from about 5 mm to        about 25 mm and such as from about 10 mm to about 20 mm as well        as a width within the range from about 1 mm to about 40 mm, such        as from about 5 mm to about 30 mm and such as from about 10 mm        to about 20 mm;    -   with a shredding unit, such as a breaker, for shredding the gel        strips into gel pieces, such as with a length within the range        of about 2.5 mm to about 25 mm, such as from about 1 mm to about        12.5 mm, a height within the range from about 0.5 mm to about 15        mm, such as from about 0.25 mm to about 7.5 mm as well as a        width within the range from about 0.5 mm to about 20 mm, such as        from about 0.25 mm to about 10 mm and    -   a “wolf” (grinding) unit, such as a wolf, having a screw and a        hole plate for example, whereby the screw conveys against the        hole plate in order to grind and crush gel pieces into gel parts        which are in one aspect smaller than the gel pieces.

An optimal surface-volume ratio is achieved hereby, which has anadvantageous effect on the drying behaviour. A gel which has beencomminuted in this way is particularly suited to belt drying. Thethree-step comminution offers a better “airability” because of the airchannels located between the granulate kernels.

In an additional aspect of the present invention, the gel granulateobtained after step ii) in the process according to the inventioncomprises to at least about 10 wt %, such as at least about 20 wt % andsuch as at least about 30 wt %, respectively based upon the total weightof the gel granulate, an absorbent polymer based on:

-   -   (α1) about 0.1 wt % to about 99.999 wt %, such as about 20 wt %        to about 97.98 wt % and such as about 60 wt % to about 93.95 wt        % of polymerized, ethylenically unsaturated, acidic        groups-comprising monomers or salts thereof, or polymerized,        ethylenically unsaturated monomers comprising a protonated or a        quaternary nitrogen, or mixtures thereof, wherein mixtures        comprising at least ethylenically unsaturated, acidic        groups-comprising monomers, such as acrylic acid, are utilized,    -   (α2) 0 wt % to about 70 wt %, such as about 1 wt % to about 60        wt % and such as about 1 wt % to about 40 wt % of polymerized,        ethylenically unsaturated monomers which can be co-polymerized        with (α1),    -   (α3) about 0.001 wt % to about 10 wt %, such as about 0.01 wt %        to about 7 wt % and such as about 0.05 wt % to about 5 wt % of        one or more cross-linkers,    -   (α4) 0 wt % to about 30 wt %, such as about 1 wt % to about 20        wt % and such as about 5 wt % to about 10 wt % of water-soluble        polymers, as well as    -   (α5) 0 wt % to about 20 wt %, such as about 0.01 wt % to about 7        wt % and such as about 0.05 wt % to about 5 wt % of one or more        auxiliaries,

respectively based on the dry absorbent polymer, wherein the sum of thecomponent weights (α1) to (α5) amounts to 100 wt %.

The gel granulate commonly comprises water in a significant amount,since the absorbent polymer is obtained from an aqueous monomersolution, which, for example, comprises water within the range fromabout 90 wt % to about 50 wt %, such as within the range from about 88wt % to about 55 wt % and such as within the range from about 70 wt % toabout 60 wt % water, respectively based on the total aqueous monomersolution.

The monoethylenically unsaturated, acid groups-comprising monomers (α1)can be partially or fully, particularly, partially neutralized. In oneaspect, the monoethylenically unsaturated, acid groups-comprisingmonomers are neutralized to at least about 25 mol %, such as at leastabout 50 mol % and such as to about 50 mol % to about 90 mol %. Theneutralization of the monomers (α1) can occur before and also after thepolymerization. Further, the neutralization can occur with alkali metalhydroxides, alkaline earth metal hydroxides, ammonia as well ascarbonates and bicarbonates. In addition, every further base isconceivable which forms a water-soluble salt with the acid. A mixedneutralization with different bases is also conceivable. In one aspect,neutralization with ammonia or with alkali metal hydroxides occurs, suchas with sodium hydroxide or with ammonia.

Furthermore, in a polymer the free acid groups may predominate, so thatthis polymer has a pH value lying in the acidic range. This acidicwater-absorbing polymer may be at least partially neutralized by apolymer with free basic groups, such as amine groups, which polymer isbasic compared to the acidic polymer. These polymers are termed“mixed-bed ion-exchange absorbent polymers” (MBIEA polymers) in theliterature and are disclosed in WO 99/34843. The disclosure of WO99/34843 is introduced here by way of reference. As a rule MBIEApolymers represent a composition that contain on the one hand basicpolymers that are able to exchange anions, and on the other hand containa polymer that is acidic compared to the basic polymer and that is ableto exchange cations. The basic polymer has basic groups and is typicallyobtained by the polymerization of monomers that carry basic groups orgroups that can be converted into basic groups. These monomers are aboveall those that have primary, secondary or tertiary amines or thecorresponding phosphines or at least two of the aforementionedfunctional groups. This group of monomers includes in particularethyleneamine, allylamine, diallylamine, 4-aminobutene, alkyloxycyclene,vinylformamide, 5-aminopentene, carbodiimide, formaldacin, melanin andthe like, as well as their secondary or tertiary amine derivatives.

The disclosures of DE 102 23 060 A1, in particular with respect to themonomers (α1) and (α2), the crosslinkers (α3) as well as in respect ofthe initiator and initiator quantities employed are referred to herein.

Monoethylenically unsaturated, acid groups-containing monomers (α1) arecited in DE 102 23 060 A1 as monomers (α1), such as acrylic acid.

In another aspect of the present invention, the absorbent polymercomprises carboxylate group-containing monomers to at least about 50 wt%, such as at least about 70 wt % and such as at least about 90 wt %,based on the dry weight. According to another aspect of the invention,that the absorbent polymer is formed from at least about 50 wt %, suchas at least about 70 wt % of acrylic acid, which is neutralized, forexample, to at least about 20 mol %, such as at least about 50 mol %.

Monoethylenically unsaturated monomers (α2) which are copolymerizablewith (α1) are utilized, which are cited in DE 102 23 060 A1 as monomers(α2), such as acrylamide.

Exemplary cross-linkers (α3) according to the invention are compoundswhich have at least two ethylenically unsaturated groups in one molecule(cross-linker class I), compounds which have at least two functionalgroups which can react with functional groups of the monomers (α1) or(α2) in a condensation reaction (=condensation cross-linkers), in anaddition reaction or a ring-opening reaction (cross-linker class II),compounds which have at least one ethylenically unsaturated group and atleast one functional group which can react with functional groups of themonomers (α1) or (α2) in a condensation reaction, an addition reactionor a ring-opening reaction (cross-linker class III), or polyvalent metalcations (cross-linker class IV). Thus with the compounds of cross-linkerclass I a cross-linking of the polymer is achieved by radicalpolymerization of the ethylenically unsaturated groups of thecross-linker molecules with the monoethylenically unsaturated monomers(α1) or (α2), while with the compounds of cross-linker class II and thepolyvalent metal cations of cross-linker class IV a cross-linking of thepolymer is achieved respectively via condensation reaction of thefunctional groups (cross-linker class II) or via electrostaticinteraction of the polyvalent metal cation (cross-linker class IV) withthe functional groups of the monomer (α1) or (α2). With compounds ofcross-linker class III a cross-linking of the polymers is achievedcorrespondingly by radical polymerization of the ethylenicallyunsaturated groups or also by condensation reaction between thefunctional groups of the cross-linkers and the functional groups of themonomers (α1) or (α2).

Exemplary crosslinkers (α3) are all those compounds which are cited inDE 102 23 060 A1 as crosslinkers (α3) of the crosslinker classes I, II,III and IV, whereby

-   -   as compounds of crosslinker class I, N,N′-methylene        bisacrylamide, polyethyleneglycol di(meth)acrylates,        triallylmethylammonium chloride, tetraallylammonium chloride and        allylpolyethyleneglycol acrylate produced with 9 mol ethylene        oxide per mol acrylic acid may be utilized, and    -   and as compounds of crosslinker class IV, Al₂ (SO₄)₃ and its may        be utilized.

In one aspect of the invention, the absorbent polymers are polymerswhich are crosslinked by crosslinkers of the following crosslinkerclasses or by crosslinkers of the following combinations of crosslinkerclasses respectively: I, II, III, IV, I III, I IV, I II, III, I II IV, IIII IV, II III IV, II IV or III IV. The preceding combinations ofcrosslinker classes represent respectively exemplary embodiments ofcrosslinkers of a polymer.

Further embodiments of the absorbent polymers are polymers which arecrosslinked by any of the crosslinkers disclosed in DE 102 23 060 A1 ascrosslinkers of crosslinker classes I, whereby N,N′-methylenebisacrylamide, polyethyleneglycol di(meth)acrylates,triallylmethylammonium chloride, tetraallylammonium chloride andallylpolyethyleneglycol acrylate produced from 9 mol ethylene oxide permol acrylic acid are utilized as crosslinkers of crosslinker class I.

The absorbent polymers can be produced from the above-named monomers andcross-linkers by various polymerization means known from the prior art.For example, in this context can be named bulk polymerization, which mayoccur in kneading reactors such as extruders or by belt polymerization,solution polymerization, spray polymerization, inverse emulsionpolymerization and inverse suspension polymerization. Solutionpolymerization, according to the invention, may be carried out in wateras solvent. The solution polymerization can occur continuously ordiscontinuously. From the prior art a broad spectrum of variationpossibilities can be learnt with respect to reaction proportions such astemperatures, type and quantity of the initiators as well as of thereaction solution. Typical processes are described in the followingpatent specifications: U.S. Pat. No. 4,286,082, DE 27 06 135, U.S. Pat.No. 4,076,663, DE 35 03 458, DE 40 20 780, DE 42 44 548, DE 43 23 001,DE 43 33 056, DE44 18 818.

As generally customary, the polymerization is started by use of aninitiator. As initiators for initiation of the polymerization allinitiators forming radicals under the polymerization conditions can beused, which are commonly used in the production of superabsorbers. Aninitiation of the polymerization by action of electron radiation on thepolymerizable aqueous mixture is possible. The polymerization can alsobe started in the absence of initiators of the above type by action ofenergetic radiation in the presence of photoinitiators. Thepolymerization initiators may be dissolved or dispersed in a solution ofmonomers according to the invention. As initiators may be used allcompounds known to the person skilled in the art that decompose to formradicals. Initiators and initiator quantities to be used are thoseinitiators and quantities respectively which are cited in DE 102 23 060A1 as initiators and quantities respectively.

A redox system comprising hydrogen peroxide, sodium peroxodisulfate andascorbic acid, for example, is used according to one aspect of theinvention. In general, according to the invention azo-compounds are usedas initiators, such as azo-bis-amidinopropane dihydrochloride. As a rulethe polymerization is initiated with the initiators in a temperaturerange from about 0° C. to about 90° C.

As water soluble polymers (α4), water soluble polymers such as thosecomprising partially or fully saponified polyvinyl alcohol,polyvinylpyrrolidone, starches or starch derivatives, polyglycols orpolyacrylic acids may be polymerized into the water-absorbing polymeraccording to the invention. The molecular weight of these polymers isnot critical, as long as they are water-soluble. Water-soluble polymersmay be starches or starch derivatives or polyvinyl alcohol. Thewater-soluble polymers, such as synthetic like polyvinyl alcohol, canalso serve as graft basis for the monomers to be polymerized.

As auxiliary (α5), suspension agents, odour binders, surface-activeagents, or antioxidants may be comprised in the absorbent polymers.These auxiliaries (α5) may be added to the monomer solution before thepolymerization, or mixed with the polymer after its production, wherebyfor the mixing, the mixing aggregates known to the skilled person can beused, e.g. the Patterson-Kelley mixer, DRAIS turbulence mixer, Lödigemixer, Ruberg mixer, snail mixer, plate mixer and fluidized bed mixer aswell as continuously running vertical mixers in which the polymers andthe auxiliary (α5) are mixed by means of rotating blades at fastfrequency (Schugi mixer).

The gel granulate dried as a polymer cake in process step iii) has atleast one of the following properties:

-   -   (φ1) the maximum absorption of an about 0.9 wt. % aqueous NaCl        solution according to ERT 440.1-99 lies within a range from        about 10 g/g to about 1000 g/g SAP granulate,    -   (φ2) the fraction that can be extracted with an about 0.9 wt %        aqueous NaCl solution according to ERT 470.1-99 is less than        about 30, based on the SAP granulate,    -   (φ3) the bulk density according to ERT 460.1-99 lies within the        range from about 300 to about 1000 g/l,    -   (φ4) the pH value according to ERT 400.1-99 of about 1 g of the        SAP granulate in about 1 liter of water is in the range from        about 4 to about 10,    -   (φ5) the CRC value according to ERT 441.1-99 lies within the        range from about 10 g/g to about 100 g/g,    -   (φ6) the AAP value at a pressure of about 0.3 psi according to        ERT 442.1-99 lies within the range from about 10 g/g to about 60        g/g.

In another aspect of the invention, at least about 30, such as at leastabout 60 and such as at least about 80 wt. % of the absorbent polymerparticles have a particle size within the range from about 150 μm toabout 850 μm. According to an additional aspect of the presentinvention, the gel granulate is to at least about 50 wt %, such as atleast about 75 wt % based on particles having a particle size in therange between about 300 μm and about 600 μm.

In additional aspects of the invention, in the context of the processaccording to the invention as well as in the context of the absorbentpolymers obtainable by the process according to the invention, thevalues of features according to the invention given only with a lowerlimit have an upper limit which is about 20-fold, such as about 10-foldand such as about 5-fold the lower limit.

In an another aspect of the invention, the absorbent polymers, such aspolymer particles, obtainable by the process according to the inventionhave at least one of the following properties:

-   -   (A) maximum absorption of an about 0.9 wt % aqueous NaCl        solution according to ERT 440.1-99 within a range from at least        about 10 g/g to about 1000 g/g, such as about 15 g/g to about        500 g/g and such as about 20 g/g to about 300 g/g,    -   (B) the fraction that can be extracted with an about 0.9 wt %        aqueous NaCl solution according to ERT 470.1-99 is less than        about 30 wt %, such as less than about 20 wt % and such as less        than about 10 wt %, based on the absorbent polymer,    -   (C) the bulk density according to ERT 460.1-99 lies within the        range from about 300 to about 1000, such as about 310 g/l to        about 800 g/l and such as about 320 g/l to about 700 g/l,    -   (D) the pH value according to ERT 400.1-99 of 1 g of the        absorbent polymer in 1 liter of water lies within the range from        about 4 to about 10, such as about 5 to about 9, and such as        about 5.5 to about 7.5,    -   (E) the CRC value according to ERT 441.1-99 lies within the        range from about 10 g/g to about 100 g/g, such as about 15 g/g        to about 80 g/g, and such as about 20 g/g to about 60 g/g,    -   (F) the AAP value at a pressure of about 0.3 psi according to        ERT 442.1-99 lies within the range from about 10 g/g to about 60        g/g, such as about 15 g/g to about 50 g/g and such as about 20        g/g to about 40 g/g.

The property combinations of two or more properties of the propertieslisted above represent respective exemplary embodiments of the polymerobtainable by the process according to the invention. Furthermore,exemplary embodiments according to the invention are processes in whichthe absorbent polymer obtained has the following properties or propertycombinations identified by letters or combinations of letter: A, B, C,D, E, F, AB, AC, AD, AE, AF, EF, ABC, ABD, ABE, ABF, ACD, ACE, ACF, ADE,ADF, AEF, CEF, ABCD, ABCE, ABCF, ABDE, ABDF, ACDE, ACDF, ACEF, ADEF,ACDEF, ABDEF, ABCEF, ABCDF, ABCDE, ABCDEF, particularly, combination CEFand combination EF.

In another embodiment of the process according to the invention, theouter portion of the absorbent polymer obtained by the process accordingto the invention, such as polymer particles, is brought into contactwith a compound comprising an Al³⁺ ion. In this case, for example, thecompound comprising Al³⁺ ions in a quantity within the range from about0.01 to about 30 wt. %, such as in a quantity within a range from about0.1 to about 20 wt. % and further such as within a range from about 0.3wt % to about 5 wt %, respectively based on the weight of the absorbentpolymer, is brought into contact with the polymers.

In this context the Al³⁺ ion-containing compound is, for example, in theform of a fluid comprising a solvent, such as water, organic solventswhich are miscible with water such as methanol or ethanol or a mixtureof at least two of these solvents, and the Al³⁺ ion-containing compound,is brought into contact with the polymer. The Al³⁺ ion-containingcompound, for example, is comprised in the fluid in a quantity, withouttaking into account water of crystallization, within a range from about0.1 to about 50 wt. %, such as in a quantity within the range from about1 to about 30 wt. %, respectively based upon the total weight of thefluid. In an additional aspect, the fluid in a quantity within a rangefrom about 0.01 wt % to about 15 wt %, such as in a quantity within arange from about 0.05 to about 6 wt. %, respectively based on the weightof the absorbent polymer, is brought into contact with the absorbentpolymer.

Exemplary Al³⁺ ion-containing compounds are AlCl₃.6H₂O,NaAl(SO₄)₂.12H₂O, KAl(SO₄)₂.12H₂O, or Al₂(SO₄)₃.14-18H₂O.

In another aspect of the invention, the polymers, such as polymerparticles, obtainable by the process according to the invention, have aninner region, an outer region surrounding the inner region and a surfaceregion surrounding the outer region, whereby the outer region has ahigher degree of crosslinking than the inner region, so that, forexample, a core-shell structure is formed. Also, in this connection theradius of the outer region may be at least twice the radius of the innerregion. The increased crosslinking in the surface region of thepolymers, such as of the polymer particles, may be achieved bypost-crosslinking of reactive groups in the area of the surface. Thispost-crosslinking can occur thermally, photochemically or chemically. Aspost-crosslinker for the chemical post-crosslinking, the compounds maybe those mentioned as crosslinkers (cc3) of crosslinker classes II andIV, such as ethylene carbonate.

The post-crosslinker may be used for post-crosslinking in an amountwithin a range from about 0.01 wt % to about 30 wt %, such as in aquantity within the range from about 0.1 wt % to about 20 wt %, such asin a quantity within a range from about 0.5 wt % to about 10 wt %,further such as in a quantity within a range from about 0.3 wt % toabout 50 wt %, respectively based on the weight of the absorbent polymerused in the process according to the invention.

The post-crosslinking may occur by bringing into contact apost-crosslinking fluid comprising a solvent, such as water, organicsolvents miscible with water such as methanol or ethanol or mixtures ofat least two thereof, as well as the post-crosslinker, with the outerportion of the polymers, such as of the polymer particles, at atemperature within a range from about 30° C. to about 300° C., such aswithin a range from about 100° C. to about 200° C. The bringing intocontact may occur by spraying the post-crosslinking fluid onto thepolymers and then mixing the polymers which have been brought intocontact with the post-crosslinking fluid. The post-crosslinker may be inthe post-crosslinking fluid in a quantity within a range from about 0.01wt % to about 20 wt %, such as in a quantity within a range from about0.1 wt % to about 10 wt %, based on the total weight of thepost-crosslinking fluid. In an additional aspect, the post-crosslinkingfluid is in a quantity within a range from about 0.01 wt % to about 50wt %, such as in a quantity within a range from about 0.1 wt % to about30 wt %, respectively based on the weight of the polymers, is broughtinto contact with the polymers.

The absorbent polymers, such as polymer particles, crosslinked asdescribed above, are referred to as “post-crosslinked polymers” or“post-crosslinked polymer particles” respectively. In another embodimentof the process according to the invention, the outer portion of thepost-crosslinked polymers, such as polymer particles, is brought intocontact with a compound comprising an Al³⁺ ion. Concerning the compoundcomprising an Al³⁺ ion, reference is made to the above details. Thecompound comprising a Al³⁺ ion may be applied to the absorbent polymerin a fluid exactly as for the other above described post-crosslinkersand then likewise thermally treated.

The post-crosslinked polymer, such as polymer particles, has at leastone, such as all of the following properties:

-   -   (N1) a CRC value determined according to ERT 441.1-99 within a        range from about 20 g/g to about 40 g/g, such as within a range        from about 25 g/g to about 35 g/g;    -   (N2) an AAP value determined according to ERT 442.1-99 at a        pressure of 0.3 psi within a range from about 20 g/g to about 35        g/g, such as within a range from about 25 g/g to about 30 g/g;    -   (N3) an AAP value determined according to ERT 442.1-99 at a        pressure of about 0.7 psi within a range from about 20 g/g to        about 27 g/g, such as within a range from about 22 g/g to about        25 g/g.

Using the particular drying process, absorbent polymers withparticularly uniform physical-chemical properties are prepared. Thedrying process allows an efficient production of absorbent polymers. Theabsorbent polymers according to the invention obtainable by theproduction process according to the invention are characterized byparticularly uniform physical-chemical properties.

The invention also relates to a composite comprising the absorbentpolymers according to the invention and a substrate. The absorbentpolymers according to the invention and the substrate may be firmlycombined with one another. As substrate may be sheets formed frompolymers, for example from polyethylene, polypropylene or polyamide,metals, non-wovens, fluff, tissues, woven fabrics, natural or syntheticfibers, or other forms.

According to the invention, composites may be sealant materials, cables,absorbent cores as well as diapers and hygiene articles, such assanitary napkins, containing these.

If the composite is an absorbent core comprising the absorbent polymerand a fiber material, absorbent polymer may be incorporated in an amountwithin the range from about 10 wt % to about 90 wt %, such as from about20 wt % to about 80 wt % and such as from about 40 wt % to about 70 wt%, based on the core. In one embodiment of the core the absorbentpolymer is incorporated as particles into the core. The polymerparticles can be distributed homogeneously in the fiber material, theycan be included in layers between the fiber material or theconcentration of the absorbent polymer particles can have a gradientwithin the fiber material. In another embodiment of the core theabsorbent polymer is incorporated into the core as fibers. In respect ofthe exact nature and structure of the absorbent core, reference is madeto the details concerning this in U.S. Pat. No. 5,562,646.

The invention further relates to a process for producing a composite,whereby an absorbent polymer according to the invention, such as apolymer structure, and a substrate and optionally a suitable auxiliaryare brought into contact with each other. The bringing into contactoccurs, in particular if the composite is a core, such as by wet laidand air laid processes, compacting, extruding and mixing.

The invention additionally relates to a composite, which is obtainableby the above process.

The invention further relates to chemical products, in particular foams,formed bodies, fibers, sheets, films, cables, sealing materials,liquid-absorbing hygiene articles, for example diapers and sanitarynapkins, carriers for plant or fungus growth regulating agents or plantprotecting agents, additives for building materials, packaging materialsor soil additives, which comprise the absorbent polymer according to theinvention or the above described composite.

The invention additionally relates to the use of the absorbent polymeraccording to the invention or of the above-described composite inchemical products, in particular in foams, formed bodies, fibers,sheets, films, cables, sealing materials, liquid absorbing hygienearticles, for example diapers and sanitary napkins, in carriers forplant or fungus growth regulating agents or plant protection agents,additives for building materials, packaging materials or soil additives.

In the use as carriers for plant or fungus regulating agents or forplant protection agents, the plant or fungus growth regulating agent orthe plant protection agent may be released over a period of timecontrolled by the carrier.

The device according to the invention for comminuting and drying apolymer according to the process according to the invention comprises acomminuting device for comminuting the undried polymer to a gelgranulate, a loosening device for loosening the gel granulate and adrying device for the gel granulate, whereby the comminuting unit, theloosening unit and the drying unit are in communicating contact witheach other. The undried polymerized polymer is comminuted with the helpof the comminuting device. With the loosening device, the bulk densityof the comminuted polymer, in particular of the gel granulate, isreduced by at least one, such as at least about 5, such as at leastabout 10, such as at least about 20 and such as at least about 30. Withthe drying device, the water content of the undried gel granulate isreduced.

Advantageously, the comminuting device comprises a cutting unit, aripping unit and a “wolf” (grinding) unit. The absorbent polymer is cutwith the help of the cutting unit. With the help of the ripping unit,the polymerized polymer is torn up, i.e. loaded under tension. With thehelp of the wolf unit, the polymerized polymer is crushed. By thecombination of these three types of comminution, a gel granulate isobtained which is particularly advantageous to dry. Advantageously, theloosening unit is a rotating drum, such as a drum, such as a drumrotation mixer. With the help of the drum rotation mixer, the averagebulk density of the gel granulate, in particular the undried gelgranulate, is reduced. The loosening effects advantageously a spreadbehaviour of the gel granulate.

The present invention also relates to a polymerization device comprising

-   -   a monomer solution container with a monomer solution conduit,    -   an initiator container with an initiator conduit,    -   a polymerization area,    -   a device according to the invention for comminuting and drying a        polymer, whereby the monomer solution conduit and the initiator        conduit are attached to an entry area of the polymerization area        and the device for drying of a polymer is arranged at an exit        area.

As monomer solution container, are considered all containers known tothe skilled person, such as plastic or steel tanks. The same is the casefor the initiator container. The conduits may be made from materialssuch as plastic or steel which are inert to the monomers or theinitiators. As polymerization area, kneading or belt polymerizationreactors may be used, particularly the latter. Absorbent polymers whichare particularly suitable for further processing by means of the processaccording to the invention can be obtained from belt polymerizationreactors. In addition, belt polymerization reactors allow a continuousflow of the process according to the invention. In knead reactors, wormshafts are used as conveying means and in band polymerization reactors,one or more conveyor belts or adjacently arranged monomer solution- andinitiator accepting forms are used, such as a conveyor belt forming adepression.

The process according to the invention for producing an absorbentpolymer is characterized by a spread behaviour of the gel granulateafter step ii). The production process of an absorbent polymer isdistinguished by a particularly efficient drying step iii), whichenables a particularly gentle and even drying of the gel granulate. Thethus obtained absorbent polymers and the products and items producedtherefrom have a particularly uniform product quality and constantphysical-chemical properties.

Further details and advantages of the invention are more closelydescribed by means of the following diagram and examples. It is shownschematically:

FIG. 1 is a schematic representation of a polymerization device with adevice for drying of a polymer; and

FIG. 2 a-2 f shows the device for determination of the compressibilityand decompressibility indices respectively; FIG. 2 a shows thecylindrical vessel into which the piston (with all additional weights)was introduced (without gel granulate in the vessel) in a side view,FIG. 2 b shows the disk of the piston in a view from above, FIG. 2 c theentire piston in a side view, FIG. 2 d the gel particles situated in thecylindrical vessel before the introduction of the piston, FIG. 2 e thegel particles loaded with the piston in the cylindrical vessel, whichwere compressed by distance x and FIG. 2 f the decompressed gelparticles inside the cylindrical vessel;

FIG. 3 shows the procedure for determination of the cross section spreadindex Q and of the spread time constant τ; FIG. 3 a shows the presslingof gel particles directly after the removal of the cylindrical vesselfrom above and FIG. 3 b the “run” pressling in cross section;

FIG. 4 shows a plot of a measurement result in a compression experiment;

FIG. 5 shows a plot of a measurement result in a decompressionexperiment;

FIG. 6 is a representation of the power consumption of the cutting millswith and without loosening of the gel particles before the drying.

In FIG. 1 is given a schematic representation of a polymerization device1 with a polymerization area 2, whereby the polymerization area 2 at itsexit area 3 is linked via a compound conduit 4 with a device 5 fordrying of a polymer. A monomer solution container 6 is connected via amonomer solution conduit 7 and an initiator container 8 is connected viaan initiator conduit 9 with the entry area 10 of the polymerization area2.

A more exact description of FIGS. 2 to 6 follows in conjunction with thetest methods and the examples.

Test Methods

ERT Methods

The ERT methods are those developed by EDANA (European Non-woven andDiaper Association), which are applied here unless otherwise described.

The determination of the first and second compressibility index κ₁ andκ₂ respectively, the determination of the first and seconddecompressibility index κ₁′ and κ₂′ respectively, and the determinationof the cross section spread index Q and of the spread time constant τ iscarried out at the temperature which the gel particles whose spreadbehaviour should be determined have at the time of determining thespread behaviour. Gel particles for which the above named parameterscannot be determined by means of the following described measurementprocess do not count as gel particles preferred according to theinvention.

First κ₁ and Second κ₂ Compressibility Index

The determination of the first and second compressibility indices (κ₁and κ₂ respectively) was carried out according to the measurement devicerepresented in FIG. 2 a to 2 g.

For a determination of the first and second compressibility indices, acylindrical vessel 11 open to the top with a volume of about 2 litrescomprising a cylindrical jacket 12 made from Plexiglas with an outerdiameter of about 120 mm, an inner diameter of about 110 mm, a wallthickness of about 5 mm and an inner cross-sectional area of about 9503mm² and a circular bottom 13 made from Plexiglas with a diameter ofabout 130 mm and a thickness of about 6 mm (see FIG. 2 a) is used. Thecylindrical jacket 12 is fixed centrally on the circular bottom 13.

A piston 14 made from polypropylene having a disk 15 with a thickness ofabout 12 mm and a rod 16 with a diameter of about 19.5 mm and a lengthof about 190 mm, which is fixed centrally on the disk 15 (see FIG. 2 aand 2 c) can be introduced into the cylindrical vessel 11. The diameterof the disk 15 is selected so that the disk 15 and thus the piston 14can glide without friction in the cylindrical vessel 11. The mass of thepiston m_(piston) amounts to about 190 g. The disk 15 has a total of 32holes 17 with a diameter of about 9.5 mm and an arrangement asrepresented in FIG. 2 b. Upon introducing the piston 14, the air in thecylindrical vessel which is beneath the disk 15 should be able to escapethrough the holes. On the side of the disk 15 facing away from the rod16 is attached centrally a circular sieve 18 made from stainless steelwith a mesh size of about 50 μm and with a diameter which corresponds tothe diameter of the disk 15. The sieve 18 thus forms the bottom of theholes 17 in the disk 15. The sieve 18 should prevent gel material beingpressed through the holes 17 upon compressing the gel granulate throughthe piston 14. The piston 14 can be loaded with different additionalmasses 19 with mass m_(i). The additional masses 19 are disk-shapedbodies made from steel with a diameter of about 100 mm and a thicknessbetween about 17 mm and about 50 mm (depending on the mass m_(i) of theadditional weight). The additional weights have furthermore in thecentre a hole 20 with a diameter of about 19.5 m. Using this hole 20 itis possible to fix the additional masses 19 onto the piston 14 bypassing the rod 16 through the hole 20, as shown in diagram 2 a. Thetotal mass m_(tot)=m_(piston)+m_(i) of the piston 14 with the additionalmasses 19 should be selectable between about 1 kg and about 6 kg. Thedisk 15 of the piston 14 further has for small, cylindrical elevations21 with a diameter of about 15 mm and a height of about 10 mm, thearrangement of which on the disk 15 is shown in FIG. 2 b. By means ofthe elevations 21 it is prevented that the additional weights 19 come tolie directly on the disk 15 (see FIG. 2 a), since this would lead toclosing of the holes 20 with the result that upon introduction of thepiston 14 into the cylindrical vessel 11 no more air can escape from thevessel.

After the cylindrical vessel 11 and the piston 14 together with thefirst used additional weight 19 have been pre-warmed to the temperatureof the gel particles whose spread behaviour should be determined (forexample by means of a corresponding temperature-controlled incubator),the vessel 11 is filled to an average fill height h of h=about 150 mmwith the gel granulate 22 of an absorbent polymer whose spread behaviourshould be determined (see FIG. 2 d). Directly after the filling, thepre-warmed piston 14 with the respective additional weight 19 isintroduced into the vessel 11 until it presses down on the gel granulate22. The time is noted at which the piston 14 with its total mass (massof the piston plus mass of the additional weight) first touches the gelparticles 22. 15 seconds after this point of time the compressiondistance x (in mm), by which the gel granulate 22 under the weight ofthe piston 14 is pressed together, is measured (see FIG. 2 e).

The piston 14 and the gel granulate 22 are then removed from thecylindrical vessel 11 and the vessel 11 as well as the piston 14 areagain pre-warmed to the temperature of the gel particles to beinvestigated. New gel granulate is placed in the vessel 11 up to anaverage fill height h of h=about 150 mm and the piston 14 with a second(e.g. heavier in comparison to the first total mass) total mass is againintroduced carefully into the vessel 11 and after 15 seconds thecompression distance x (in mm), by which the gel granulate has beenpressed together unto the weight of the piston 14, is measured again. Inthis way the respective compression distances for the remaining totalmasses are determined. From the respective compression distances x andthe fill height (about 150 mm) a respective normalized volume reductionfor each total mass is determined as a percentage, given by ΔV/V₀=x/h.The normalization is based on the original, uncompressed volume V₀. Fromthe respective total masses m_(tot) and the inner cross-sectional areaA′ of the cylindrical vessel 11, the compression pressure Δp for therespective total masses is determined Δp=m_(tot)·9.81 m/s²/A. Thenormalized volume reduction ΔV/V₀ for the different total masses isplotted in a grass against the respective compression pressure Δp. Aregression line, which is determined using a linear regression, is drawnthrough the values of the normalized volume reduction. The linearregression is determined for compression pressures in the intervalbetween about 1000 Pa and about 7000 Pa. The gradient of the regressionline is the second compressibility index κ₂. The offset of the line isthe first compressibility index κ₁ (see FIG. 4).

First κ₁′ and Second κ₂′ Decompressibility Index

The decompression experiment follows the compression experiment. Thesame measurement device is used as for the determination of thecompression experiments.

The gel granulate 22 is first compressed as described under the weightof a piston 14 which has a first total mass and then decompressed, i.e.relaxed. This occurs when the piston 14 with the additional mass m_(i)is removed from the gel granulate 22. Through the thus affected release,the compressed gel granulate relaxes by a decompression distance x′(inmm) see FIG. 2 f). It is proceeded in such a way that the gel granulate22 is placed in the vessel 11 which has been pre-warmed to thetemperature of the gel granulate up to an average fill height h of about150 mm and directly after the filling of the likewise pre-warmed piston14 together with the additional weight 19 is introduced into the vessel11. After the piston 14 has pressed on the gel granulate 22 for 15seconds, the piston 14 is removed and a further about 15 seconds afterremoval of the piston 14, the decompression distance x′ is determined(see FIG. 2 f). The gel granulate 22 is then removed from thecylindrical vessel 11, new gel granulate 22 is placed in the vessel 11which has been pre-warmed again up to an average fill height h ofh=about 150 mm. The piston 14 with a second (e.g. heavier compared tothe first total mass) total mass carefully introduced into the vessel11, so that the gel granulate 22 is compressed. After removal of thepiston 14, the gel granulate 22 is decompressed and 30 seconds afterremoval of the piston 14, the decompression distance x′ (in mm)corresponding to the second total mass is measured. In this way, therespective decompression distances for all remaining total masses aredetermined. From the respective decompression distances, a normalizedvolume increase ΔV′/V1 (unit: percent) by ΔV′/V1=x′/(h−x) determinedrespectively for the respective total masses. The normalization here isbased on the volume V1 compressed previously by the piston 14. Thenormalized volume increase deltaV′/V1 is plotted against the compressionpressure release Δp′=m_(tot)·9.81 m/s²/A in the range of values betweenabout 1000 Pa and about 7000 Pa. A regression line, determined by linearregression, is drawn through the points. The second decompressibilityindex κ₂′ is the gradient of the regression line. The firstdecompressibility index κ₁′ is the offset of the line (see FIG. 5).

Cross Section Spread Index Q and the Spread Time Constant τ

The cross section spread index Q and the spread time constant τ aredetermined by a so-called putting-over-experiment. To this end, a deviceis used which corresponds to the device described in the context of thedetermination of the compression and decompression index respectively,but with the difference that the cylindrical vessel 11 has no circularbottom 13 and is thus open to the downside.

For the determination of the cross section spread index Q and of thespread time constant τ, the cylindrical vessel 11 is placed on a flatsubstrate plate 24 made from polypropylene, so that this substrate plate24 forms the bottom of the cylindrical vessel 11. The cylindrical vessel11 and the substrate plate 24 have been previously pre-warmed to thetemperature of the gel particles whose spread behaviour should bedetermined. The gel particles are then placed in the cylindrical vessel11 to an average fill height of about 150 mm and directly afterwards thelikewise pre-warmed piston 14 together with a first, likewise pre-warmedadditional weight 19 placed on the gel particles. About 30 secondslater, the piston 14 is removed from the cylindrical vessel and thecylindrical vessel is lifted so that a cylindrical pressling 23 of gelparticles remains on the substrate plate 24 (see FIG. 3 a).

In this experiment, the spread behaviour is shown in that the pressling23 is no longer held in shape as soon as the cylindrical vessel 11 islifted and distributes itself over a comparably large area of thesubstrate plate 24 (see FIG. 3 b). The distribution of the gel granulateon the level surface takes a few seconds. The spread behaviour of thegel granulate or of the loosened gel granulate is determined by a spreadtime constant τ. The spread time constant τ is determined by the timewhich elapses until the gel granulate shaped into a cylindricalpressling ends its spread movement, after which the pressling is nolonger held in shape. The cross section spread index is determined inthat after the end of the spread movement (the spread movement is endedwhen at an arbitrary boundary point of the spread pressling within atime interval of about 30 seconds no further spreading can be observedwith the eye) the area covered by the gel granulate (=F′) is determined.The cross section spread index is defined as follows: Q=F′/F, whereby Fis the cross-sectional area of the pressling before the raising of thecylindrical vessel 11 (F=9503 mm²).

COMPARATIVE EXAMPLE

composition (amounts in kg for input 1): 4000.0 water 2030.0 50% sodiumhydroxide 2610.0 acrylic acid 105.0 methoxypolyethylene glycol (17 EO)methacrylate 15.7 polyethyleneglycol (10 EO) allyletheracrylate 8760.7monomer solution for input 1

About 400 kg/h of this monomer solution are cooled in a heat exchangerto about 1° C. and freed from dissolved oxygen to a residual content ofabout 0.9 ppm in a stripper with a throughflow of about 3m³/h nitrogen.The following solution quantities were mixed with this input 1 beforefeeding onto the polymerization belt:

input about 2: 8.8 l/h 0.75% sodium peroxodisulfate solution

input about 3: 8.8 l/h2,2′-azobis(2-methylpropionamidine)dihydrochloride

input about 4: 8.8 l/h 0.5% hydrogen peroxide solution

input about 5: 8.8 l/h 0.075% ascorbic acid solution

input about 6: 10 kg/h of a solution of about 15 kgpolyethyleneglycol-400 dimethacrylate in about 200 kg water.

After a dwell time of about 40 minutes, the solid, still hot polymergel, is comminuted using a “Fleischwolf” (meat grinder) to a gelparticle size within a range of about 150 μm to about 3000 μm(pre-comminution) and dried on a belt dryer in zone 1 and 2 at about160° C., zone 3 with about 140° C. and zone 4 and 5 with about 130° C.delivery air temperature. The dried gel granulate is then ground on acutting mill attached after the belt dryer. The power consumption of thecutting mill during the grinding is seen in the diagram shown on FIG. 6(see the right half of the diagram in FIG. 6: power consumption “withoutgel loosening”).

EXAMPLE 1

The comparative example is repeated. After the gel comminution andbefore the drying on the belt dryer the gel particles are loosened in adrum (length: about 300 cm, diameter: about 80 cm), which turns with aspeed of about 7.1 rotations per minute about the longitudinal axis,from about 2 to about 3 minutes. The drying and grinding of the gel thenfollows, as was described in the comparative example. The powerconsumption of the cutting mill during the grinding is shown in thediagram depicted in FIG. 6 (see left half of the diagram in FIG. 6:power consumption “with loosening”). It can be seen from FIG. 6 that theloosening of the gel particles before the drying leads to a noticeablereduction in the power consumption of the cutting mill with which thedried gel particles are ground.

Example 2

The spread behaviour is determined for the gel particles obtained inexample 1 after the loosening and before the drying, which have atemperature of about 62° C.

Compression and Decompression Behaviour:

In the compression/decompression experiments it is seen that thecompression movement is stronger than the decompression movement (i.e.the value of the compression distance x is larger than the decompressiondistance x′). In the following Table 1, are represented respectively twoexperimentally obtained values for an original fill height h=about 150mm: TABLE 1 Compression distance Decompression distance Total mass [g] x[mm]¹⁾ x{grave over ( )} [mm] ¹⁾ 1185 30 16 2175 50 22 3185 53 35.5 618566 35¹⁾ Average value from 5 measurements

Cross Section Spread Index Q and Spread Time Constant τ

The results of an experiment to determine the cross section spread indexand to determined the spread time constant τ of the thus loosened gelparticles are represented in the following Table 2. In addition to thearea F′ the average height h′ of the pressling after the end of thespread movement is also determined. The determination of the spread areawas carried out by using graph paper which was laid under thetransparent substrate plate. TABLE 2 Total mass [g] Area F{grave over( )} [mm²] ¹⁾ Height h{grave over ( )} [mm]¹⁾ 1185 74375 (Q = 7.8) 402175 71375 (Q = 7.5) 35 3185 68750 (Q = 7.2) 35 6185 62500 (Q = 6.6) 32¹⁾ Average value from 5 measurements

In FIG. 4, the normalized volume reduction caused by the compression isplotted against the compression pressure. A regression line was drawnthrough the measured points. The gradient of the line is about 4.12×10⁻⁵Pa⁻¹. The offset of the line is about 21%.

In FIG. 5, the normalized volume increase is plotted against thedecompression pressure. A regression line is drawn through the measuredvalues. The gradient of the line is about 5.9×10⁻⁵ Pa⁻¹; the offset ofthe line is about 6.4%. It can be seen that the decompression, i.e.after removal of the piston 11 with the additional masses 19, is lessstrong than the compression. In particular, the compression differs fromthe decompression by different offsets, whereas the gradients of theregression line within the measurement error bars are the same.

FIG. 6 shows the power consumption of the cutting mill with and withoutthe process according to the invention, i.e. with and without looseningof the gel particles. The cutting mill is connected after the device fordrying 5 and serves to comminute the dried polymer.

The cutting mill connected after the belt dryer shows, as a sign of ahomogenous drying as a result of the loosening carried out before thedrying, a uniform electricity/power consumption.

LIST OF REFERENCE NUMERALS

-   1 polymerization device-   2 polymerization area-   3 exit area-   4 compound conduit-   5 device for drying-   6 monomer solution conduit-   7 monomer solution container-   8 initiator container-   9 initiator conduit-   10 entry area-   11 cylindrical vessel-   12 cylindrical jacket-   13 circular bottom-   14 piston-   15 disk-   16 rod-   17 hole in the disk 15-   18 sieve with a mesh size of 50 μm-   19 additional masses-   20 hole in the additional masses-   21 elevation-   22 gel granulate-   23 pressling made from gel granulate-   d in a diameter of the cylindrical vessel 11-   d′ outer diameter of the piston 14-   A′inner cross-sectional area of the piston 14-   x compression distance-   x′ decompression distance-   h fill height of the gel granulate 22 in the vessel 11 before the    compression experiment-   h′ final fill height of the gel granulate 22 in the vessel 11 after    decompression experiment-   F cross-section area of the pressling-   F′ area of the “flowed” pressling on the substrate plate after    spreading

1. A process for preparation of an absorbent polymer, comprising thesteps of: i) polymerizing of an aqueous monomer solution to obtain apolymer gel, ii) comminuting the polymer gel to obtain a polymer gelgranulate, and iii) drying the polymer gel granulate, wherein thepolymer gel granulate after step ii) has a spread behavior including afirst compressibility index κ₁ of the gel granulate lies between about10% and about 40%.
 2. The process according to claim 1, wherein thepolymer gel is loosened after the comminution in step ii), in the formof the polymer gel granulate obtained after the comminution, and beforethe start of the drying step iii) and a mechanically loosened polymergel granulate is obtained.
 3. The process according to claim 1, whereinthe spread behavior of the gel granulate after step ii) is characterizedby at least one of the following features: (δ1) a second compressibilityindex κ₂ of the gel granulate lies between about 3·10⁻⁵ Pa⁻¹ and about6·10⁻⁵ Pa⁻¹; or (δ2) a first decompressibility index κ₁′ of the gelgranulate lies between about 3% and about 15%; or (δ3) a secondcompressibility index κ₂′ of the gel granulate lies between about 3·10⁻⁵Pa⁻¹ and about 7·10⁻⁵ Pa⁻¹.
 4. The process according to claim 1, whereinthe spread behavior of the gel granulate or of the loosened gelgranulate after step ii) is characterized by a cross section spreadindex Q of at least about 3 with a load with a total mass of about 1185g .
 5. The process according to claim 1, wherein the spread behavior ofthe gel granulate or of the loosened gel granulate after step ii) isdetermined by a spread time constant τ of at least about 2 s with a loadwith a total mass of about 6185 g.
 6. The process according to claim 1,wherein the density of the undried gel granulated after step ii) is lessthan about 0.7 g/cm³.
 7. The process according to claim 1, wherein thegel granulate in step ii) is subjected to an at least three-stepcomminution with a cutting unit, a ripping unit and a “wolf” (grinding)unit.
 8. The process according to claim 1, wherein the polymer gelgranulate comprises at least about 10 wt. %, based on the gel granulate,an absorbent polymer based upon: (α1) about 0.1 to about 99.999 wt. % ofpolymerized, ethylenically unsaturated, acidic groups-comprisingmonomers or salts thereof, or polymerized, ethylenically unsaturatedmonomers comprising a protonated or a quaternary nitrogen, or mixturesthereof, (α2) 0 to about 70 wt. % of polymerized, ethylenicallyunsaturated monomers which can be co-polymerized with (α1), (α3) about0.001 to about 10 wt. % of one or more cross-linkers, (α4) 0 to about 30wt. % of water-soluble polymers, as well as (α5) 0 to about 20 wt. % ofone or more auxiliaries, respectively based on the dry absorbentpolymer, wherein the sum of the component weights (α1) to (α5) amountsto 100 wt. %.
 9. The process according to claim 1, wherein the dried gelgranulate has at least one of the following properties: (φ1) the maximumabsorption of a 0.9 wt. % aqueous NaCl solution lies within a range fromabout 10 to about 1000 g/g SAP granulate, (φ2) the fraction that can beextracted with a 0.9 wt. % aqueous NaCI solution is less than about 30,based on the SAP granulate, (φ3) the bulk density lies within the rangefrom about 300 to about 1000 g/l, (φ4) the pH value of about 1 g of theSAP granulate in about 1 litre of water is in the range from about 4 toabout 10, (φ5) the CRC value lies within the range from about 10 toabout 100 g/g, (φ6) the AAP value at a pressure of about 0.3 psi lieswithin the range from about 10 to about 60 g/g.
 10. An absorbent polymerparticle obtained according to the process of claim
 1. 11. A compositecomprising the absorbent polymer particles obtained according to claim10, and a substrate.
 12. A process for production of a composite,wherein the absorbent polymer particles according to claim 10 and asubstrate and optionally an auxiliary are brought into contact with eachother.
 13. A chemical product comprising the absorbent polymer particleaccording to claim
 10. 14. (canceled)
 15. A device for carrying outprocess steps ii) and iii) of the process as defined in claim 1,comprising a comminution device for comminution of the undried polymerto a polymer gel granulate, a loosening device for loosening for the gelgranulate and a drying device for the gel granulate, wherein thecomminution device, the loosening device and the drying unit are incommunicating contact with each other.
 16. The device according to claim15, wherein the comminution device comprises a cutting unit, a rippingunit, and a “wolf” (grinding) unit.
 17. The device according to claim15, wherein the loosening unit is a rotating drum.
 18. A polymerizationdevice comprising: a monomer solution container with a monomer solutionconduit, an initiator container with an initiator conduit, and apolymerization area, wherein at an entry area of the polymerization areathe monomer solution conduit and the initiator conduit are attached andat an exit area the device for carrying out process steps ii) and iii)is arranged.